1. Field of the Invention
This invention relates to a preparing method for coating PMMA (polymethylmethacrylate) particles with silicon dioxide and more particularly relates to a preparing method for complex particles with optical diffusion and villiform structure by coating PMMA particles with silicon dioxide.
2. Description of the Prior Art
As photo-electronic industry develops and innovates, various projectors and liquid crystal displays (LCDs) are common displays in families or commerce occasions gradually. The electronic apparatuses for projecting light are often applied to light-guiding plates or lenses for propagating light. At the earliest, the light-guiding plates or lenses are made of glass. However, it is unavoidable to encounter a bottleneck in mass production using such material of glass; the weight and the abrasion resistance are therefore limited. Recently, chemical polymerized material is used for new light-guiding devices in place of glass. It is applied to various optical productions, such as light-guiding members for LCD, front panels for plasma display panel (PDP), and project screens.
For example of LCD, liquid crystal itself does not light. There is a backlight as the light source. In practical applications, the light-guiding plate for notebook is formed by injecting PMMA. The light-guiding plate guides the light form a cold cathode fluorescent lamp (CFEL) or a light-emitting diode (LED) to the surface of the liquid crystal. The absorption of the light in the propagation in the light-guiding plate is much less so as to reduce the single loss to the minimum. In another aspect, the LCD panels used in the application of liquid crystal television are a little different to those used in notebooks or desktops. Especially, a large-size liquid crystal television does not use an edge light source but a direct-light backlight. Multiple CFFLs are provided for light so as to achieve the requirement of high brightness and high contrast. The light from the direct-light backlight need to pass through a layer of diffuser plate for preventing the non-uniform distribution of the light.
In general, the early diffuser plate for guiding light made of PMMA or PS (polystyrene) diffusive particles are most seen. Please refer to FIG. 1. FIG. 1 is a diagram of an SEM (scanning electron microscope) picture of PMMA particles in the prior art. The preparing method for the PMMA diffusive particles is performed by dissolving a PVP (polyvinyl pyrrolidone) dispersant, MMA (methylmethacrylate) monomers and an AIBN (azobisisobutyronitrile) initiator in the solvent of ethyl alcohol and water to obtain a solution. The solution reacts under 70 degrees in Celsius for six fours. After the reaction is completed, the solution is cooled to obtain dispersive polymer. The dispersive polymer is then centrifugally rotated and precipitates. The PMMA particles are washed by absolute alcohol several times to remove un-reacted monomers and lower polymer and then dried in vacuum for 24 hours to obtain the PMMA diffusive particles (as shown in FIG. 1). A diffuser plate coated with the conventional diffusive particles has the advantages of less weight, high abrasion resistance, anti-electrification, and anti-reflection.
However in another aspect, the conventional diffusive particle has disadvantages of low rigidity, high hygroscopicity and yellowing problem due to high temperature process environment. In practical applications in photo-electronic industry, the conventional diffuser plate coated with the diffusive particles (PMMA, PS, PC) encounters its limit. A replacing material is needed. In the prior art, the diffuser agent is macromolecular microspheres. The penetrability is good, while the heat resistance, the weathering, and the dimensional stability are poor than the inorganic diffusive particles. For the goals of high penetrability, high haze and high luminance, an organic-inorganic complex is used to make new diffusive particles recently. The glass transition temperature (Tg), the refractivity, the optical loss, and the thermal expansion coefficient could be adjusted on the content proportion of the organic-inorganic complex. In practical applications, the organic-inorganic complex is usually formed by mixing PMMA and silicon dioxide.
If inorganic material is added into hydrophobic organic macromolecular material, the inorganic material needs to be treated by an interface modification process first so as to overcome the problems of compatibility and coagulation due to the hydrophile surface of the inorganic material. For the surface modification techniques of inorganic material, there are many studies, such as U.S. Pat. Nos. 3,451,838 and 2,404,457 which disclose a coated layer mainly consisting of alkoxysilane hydrate and colloidal silicon dioxide. U.S. Pat. No. 4,877,451 discloses a surface modification on silicon dioxide particles by a coupling agent of hydroxylalkylsilane and aminoalkylsilane, so that organic colorant is bound to the surface of the silicon dioxide particles. The above patents disclose the interface modification to inorganic particles, not the relative means and effect of the complex process of inorganic particles and organic particles.
For organic-inorganic complex particles, Taiwan patent no. I255826 discloses silicon dioxide particles processed by an interface modification. The silicon dioxide particles could be mixed and dispersed in organic macromolecule so as to enhance the mechanical properties of the complex. For the preparing the complex coating material of PMMA and silicon dioxide, U.S. Pat. No. 6,871,994 discloses coating material formed by mixing macromolecule microspheres into an adhesive. U.S. Pat. No. 6,888,663 discloses a diffusive film formed by mixing only clay as diffusive particles which are first processed by an interface modification into macromolecule. That is, in the prior art, most of the complex of PMMA and silicon dioxide is prepared by mixing. The method thereof is to mix acrylic monomer MMA, silane coupling agent and silicon dioxide to a film; the execution thereof is to mix and disperse silicon dioxide particles into organic macromolecule. The structure of the above complex particles is formed by further polymerizing the silicon dioxide MMA particles; that is, the silicon dioxide particles mixed and dispersed in both the interior and the exterior of the whole complex particle. Although the complex particles formed by such method has improved mechanical properties, because the silicon dioxide particles are mixed in the whole complex particle and holes are easily present in the film of silicon dioxide particles so that the silicon dioxide particles could not dispersed uniformly, the film is easily formed with cracks thereon and the improvement to the optical properties is poor.
In another preparation of film, TEOS (tetraethoxy silane), acrylic monomers, a coupling agent, and an initiator are thermally polymerized to form macromolecule. However, the curing temperature is lower, so more silicon hydroxyl remains so that the pre-polymer before coating is unstable and easy to gel and uniform coating of silicon dioxide on the surface of the microspheres occurs easily. In addition, the problem of non-uniform particle size of the silicon dioxide particles inside the film also affects the optical homogeneity of the film.
At present, for solving the disadvantages in the preparation of the above inorganic-organic macromolecule, silicon dioxide is usually regarded as filler for the macromolecule, and silicon dioxide and the macromolecule are inorganic-organic mixed to enhance the thermal stability and the mechanical properties. The invention, from the material for an optical diffusive film, is to modify the complex particle itself and to adsorb nano-sized silicon dioxide particles on the surface of the PMMA microspheres, so that the surface of the microsphere forms a rough villiform or irregular structure capable of multiply optically reflecting and refracting and has a better optical diffusive property than the original smooth surface of the microsphere. The invention therefore overcomes the disadvantages of the non-uniform distribution of the diffusive film and easily producing holes in the process of mixing silicon dioxide particles.
For macromolecule diffusive particles or macromolecule-inorganic complex diffusive particles in the convention, the sphere form and the particle size distribution thereof affect its optical diffusive distribution. For a single sphere, the often-seen complex particle could be an inorganic-organic complex particle with the core-shell interior structure and an organic-inorganic complex particle with villiform structure. The organic-inorganic complex particles with villiform and core-shell structure of the invention is formed by interface modifying the surface of the macromolecule particle to further adsorb silicon dioxide particles to improve the optical properties thereof. Compared with the prior art from the view of the diffusion effect of optical design, the diffusion effect of the villiform and core-shell structure is better than that of single macromolecule particle.
The invention discloses a new preparing method for preparing chemical complex particles with villiform structure for optical diffusion, so as to solve the above problem.